This invention relates to pen plotters and sensors used therein for monitoring the functions thereof and, more particularly, in a pen plotter having a plotting head carrying one half of a pen exchanging mechanism including a holding plate pivotally attached to a plotting head for pivotal movement to raise and lower a pen gripped therein and carried thereby and a spring-biased gripping finger pivotally carried by the holding plate for releasably holding a pen, to a dual-function sensing apparatus for providing a single output signal indicating pen presence and pen up/down position; an optical sensor comprising a light source and a light detector separated from the light source by a space and having a sensing surface upon which a beam of light impinges and wherein the level of signal output is a function of the amount of light impinging on the sensing surface; a light blocking blade for selectively blocking portions of the beam of light; and, an activation linkage connected between the gripping finger and the light blocking blade for selectively blocking portions of the beam of light with the light blocking blade so that with no pen being held the signal output from the light detector is a first possible value, with a pen being held in a lowered position the signal output is a second possible value, and with a pen being held in a raised position the signal output is a third possible value.
In a pen plotter 10 having the portions depicted in FIG. 1, a pen 12 is releasably and exchangably held by a plotting head 14 moving back and forth on a beam 16 as indicated by the arrows 18. To effect line drawing and movement of the plotting head 14 from one vector position to another without drawing, the pen 12 is raised and lowered as indicated by the arrow 20. Typically, there is a pen exchanging station 22 at one end of the beam 16 to which the plotting head 14 can go to pick up a pen 12. Whether the pen exchanging station 22 has one pen 12 or a plurality of pens 12 held in a turret is not important. As depicted in FIG. 2, the pen exchanging station 22 includes one half of a pen exchanging mechanism, 24, and the plotting head 14 carries the other one half of the pen exchanging mechanism, 24'. The two halves of the pen exchanging mechanism 24, 24' each have a holding plate 26 containing a partial cylindrical groove 28 into which the pen 12 fits and a gripping finger 30 pivoted on its inner end and spring biased towards the groove 28. It is characteristic of the two halves of the pen exchanging mechanism 24, 24' that when one is holding a pen 12 and the two are brought together, the pen is exchange from the one to the other.
In prior art pen plotters employing such a mechanism, two sensors are employed to provide signals to the control logic (not shown) of the plotter 10. One sensor detects whether or not a pen 12 is being held by the other one half of the pen exchanging mechanism, 24' of the plotting head 14 so that plotting will not be commenced if there is no pen 12 with which to plot. The other sensor is positioned to detect when the pen 12 is in its raised and lowered positions so that the pen 12 will not be dragged over the plot in a lower state between vectors and so that plotting of a vector will not be commenced if the pen 12 has not yet achieved its lowered position in contact with the plotting media.
As with any electro-mechanical type of device, more sensors mean higher cost of manufacture and more opportunity for failure.
Wherefore, it is the object of this invention to provide a dual function sensor for a pen plotter which will sense both pen up and down as well as pen presence in the plotting head.